Environmental Engineering Reference
In-Depth Information
establish a water circulation loop just like the borehole systems connected with a
fractured reservoir created by hydraulic stimulation. The implementation of reinjec-
tion should be useful to mitigate and reduce declining heat production in the hydro-
thermal system provided that the conditions of reinjection (such as temperature and
flow rate) and its location are properly designed with respect to the heat extraction
rate. The simulation code, FRACSIM-3D as described previously may offer a use-
ful tool for the design of such reinjection strategies.
In addition to numerical simulation code, a mathematical model based on frac-
tional derivatives has been investigated in order to characterize subsurface mass and
fluid flow in inhomogeneous and complex rock masses (Fomin et al.
2005
,
2011
).
Numerical flow models which explicitly account for heterogeneities such as natural
fractures often require detailed knowledge of the subsurface structure and/or sys-
tematic data sets of field flow testing in order to determine the spatial distribution
of heterogeneities. Execution of numerical simulations based on such a flow model
also requires relatively large computer capacity and long computation times. The
fractional derivatives based mathematical model is expected to provide a simple
and useful tool to analyze the mass and heat transport in complex reservoir systems.
It is often pointed out that the flow behaviour in the vicinity of wellbores domi-
nates the overall flow impedance of the hydrothermal system because of its con-
centrated and high flow rate. Mineral depositions and plugging may be anticipated
to take place along the flow path at wellbores, reducing permeability. Thus, an im-
provement in permeability near the wellbore is expected to enhance greatly the
fluid conductance of the reservoir. Hydraulic stimulation of wellbores may provide
a very effective approach for this purpose. The operation of hydraulic stimulation is
expected to increase the aperture of pre-existing fractures intersecting the wellbore
and to connect the wellbore with isolated natural fractures, leading to enhanced wa-
ter permeability in the vicinity of the wellbore. Hydraulic stimulation may be useful
not only for the stimulation of production wells, but also in reinjection wells. Thus,
the technology developed for creating man-made reservoir systems and relevant nu-
merical simulation models can be utilized to combine the existing reservoir with an
artificial fracture system in order to mitigate the decline in the heat production. This
strategy would reduce the uncertainty associated with the development of many
geothermal energy extraction systems.
6.7
Conclusion
It has been said that the underground (or 'inner space') just below us is less studied
than the universe. One of the primary reasons for this may be its uncertainty due
to the complexity of the subsurface structure including the complicated distribu-
tion of natural fractures. Nonetheless the underground provides a safe and resilient
space for energy systems which can be truly harmonized with the environment.
This chapter describes a methodology for the development of geothermal energy
extraction with a special focus on designs for engineered geothermal reservoirs. It
